J. T. Gosling LASP / University of Colorado Boulder, Colorado, USA Comparative Aspects of Magnetic Reconnection in the Solar Wind and in Earth’s Magnetosphere J. T. Gosling LASP / University of Colorado Boulder, Colorado, USA November, 2008 Savannah, Georgia

Magnetic Reconnection 1) Occurs at thin current sheets. 2) Is a process that converts magnetic energy to bulk flow energy and plasma heating. 3) Changes the field topology. 4) Occurs throughout the heliosphere in a variety of contexts. 5) Is of fundamental importance for understanding the dynamics of the solar atmosphere and planetary magnetospheres. Does not require anti-parallel fields - only anti-parallel field components.

The Smoking Gun for Reconnection at the Magnetopause Observations obtained during a crossing of the magnetopause tailward of the dawn-dusk terminator. V Direct evidence for reconnection first came from in situ observations of the plasma jets or “exhausts” at the magnetopause as predicted by models of the reconnection process. Bx

Some Results From In Situ Observations of Reconnection at the Magnetopause and in the Magnetotail Both quasi-stationary (accelerated flows) and transient (flux transfer events) reconnection are apparently observed. Field shear angles of 180˚ are not required or even necessarily preferred at the dayside magnetopause, but are preferred along the flanks of the magnetosphere. X-line lengths are generally uncertain; reconnection is often characterized as being patchy in both space and time. Hall field effects are apparent near to and in the diffusion region during reconnection in the magnetotail. There is evidence for slow shocks during reconnection in the magnetotail, but not at the magnetopause.

Some Results From In Situ Observations of Reconnection at the Magnetopause and in the Magnetotail (continued) Plasma acceleration is confined to the field rotation region along the magnetopause flanks, but is not so confined in the sub-solar region. Reconnection prefers low beta at the magnetopause. Bifurcated current sheets are apparently produced by reconnection in the distant (50-125 Re) magnetotail at least some of the time. Reconnection in the near tail region (10-20 Re) is often characterized as “explosive”. Particle (electron) acceleration apparently occurs within the diffusion region during reconnection in the magnetotail. There is no strong evidence for particle acceleration associated with reconnection at the magnetopause.

An Asymmetric Reconnection Exhaust in the Solar Wind Np 64-s plasma data Tp 16-s field data Vn B VA Bn  Vn Roughly Alfvenic accelerated plasma flow filling field reversal region. Bifurcated current sheet (i.e. double-step field rotation). Anti-correlated changes in V and B at one edge and correlated changes in V and B at other edge.

Reconnection Exhausts in the Solar Wind 2D projection Br Vr Bt Vt Bn Vn Changes in V and B that are anti-correlated at one edge and correlated at the other edge of an accelerated flow event indicate the flows are bounded by Alfvenic disturbances propagating in opposite directions along the field lines. WARNING: It has recently been suggested that such events are more likely the result of colliding rotational discontinuities propagating in opposite directions along the magnetic field rather than the result of reconnection.

Interpenetrating Proton Beams Within Reconnection Exhausts Protons - Plasma Rest Frame Before Exhaust After Exhaust 11/23/1997 Interpenetrating proton beams demonstrate magnetic connection across an exhaust and, although not always well resolved, are characteristic of these events. Observed temperature increases are associated with these interpenetrating beams.

An Anti-Sunward-Directed Exhaust at the Heliospheric Current Sheet Suprathermal Electron Strahl Reconnection is relatively rare at the HCS but produces interesting effects when it does occur there.

Evidence for Magnetic Disconnection From the Sun Strahl disappearance and an asymmetric halo within the exhaust and separatrix layers demonstrate magnetic connection across the HCS and magnetic disconnection from Sun.

Evidence for Formation of Closed Field Lines via Reconnection Counterstreaming strahls within a sunward-directed reconnection exhaust at the heliospheric current sheet demonstrate formation of closed field lines and magnetic connection across the HCS.

Multiple Spacecraft Observations of Reconnection Jets in the Solar Wind Multi-spacecraft observations reveal prolonged reconnection, oppositely directed plasma jets, and continuous and greatly extended X-lines in at least some events.

A Very Narrow Reconnection Exhaust 3-s resolution data 92-ms resolution data Bt Br Bn BN BL BM Exhaust was 25.4 c/pi wide. Clear bifurcated current sheet. Strong guide field (BM). |BM| increased in exhaust. No Hall field rotations obvious.

Reconnection Exhausts Observed by Wind in March 2006 1-hr avgs. V B   B shear angle: 30˚ 74˚

Some Statistics for the March 2006 Reconnection Exhausts V Beta Beta Reconnection: Is quite common in the low-speed wind. Commonly occurs at small field shear angles. Commonly produces relatively narrow exhausts. Occurs preferentially in low beta plasma.

3D Structure of a Reconnection Exhaust with a Strong Guide Field

Summary of Solar Wind Observations Reconnection in the solar wind: Occurs at thin current sheets that typically separate distinctly different plasma states (tangential discontinuities). Commonly is quasi-stationary and typically occurs at extended X-lines. Produces Petschek-type exhausts of roughly Alfvenic jetting plasma bounded by back-to-back rotational discontinuities that bifurcate the original thin current sheet. Occurs frequently in the low-speed wind (40-70 events/month at 1 AU) and within ICMEs. Occurs less frequently in the turbulent, high-speed wind where the exhausts are most often embedded within outward propagating Alfvenic fluctuations.

Reconnection in the solar wind: Commonly occurs for field shear angles considerably < 90˚ (strong guide fields). Events occurring at small field shear angles tend to have smaller widths than those occurring at large field shear angles. Often appears to be undriven (spontaneous). Commonly occurs in low beta plasma. Usually is not associated with slow-mode shocks. Usually does not produce electron heating. Usually does not (never?) produce energetic particles. Is “fast” but not “explosive” - magnetic energy release occurs over a long interval following reconnection.

Questions, Problems, Topics for Future Work ** Have we learned how to identify reconnection exhausts correctly in the solar wind? Sweet-Parker vs Petschek-type reconnection - when and where? Can reconnection be initiated spontaneously or must it be driven? What sustains reconnection? What turns it off? What determines whether reconnection is quasi-stationary or transient? What characterizes explosive reconnection? Why does reconnection prefer low beta conditions? How are long X-lines formed? Are slow mode shocks a necessary aspect of fast reconnection in a collisionless plasma?

Questions, Problems, Topics for Future Work (continued) Does reconnection necessarily produce particle acceleration? If so, does the acceleration occur during reconnection or as an after-effect. What determines reconnection rates? How important is reconnection in dissipating current sheets? Are essentially all current sheets (< ~3 c/pi) disrupted by reconnection? How does reconnection work in the presence of a significant guide field? Can there be field annihilation without reconnection? We can learn much about the reconnection process by comparing observations of reconnection and its after-effects in quite different plasma environments.

The End Reprints and preprints on reconnection in solar wind (15 available): jack.gosling@lasp.colorado.edu